Wavelength division multiplexing (WDM) is pervasive in optical communication systems. Key optical components in these systems are those that perform the function of combining (multiplexing) various wavelength channels and splitting (demultiplexing) those channels. The multiplexing of various wavelength channels in current WDM systems is considered a relatively simple task and can be achieved with a component such as a star coupler.
Demultiplexing, however, requires optical spectral filters and is considered a much more challenging aspect of WDM systems when real system constraints are applied. In optical spectral filters, ideally, signals in the passband should emerge undistorted and other signals should be rejected. This implies an infinitely steep passband edge, a flat passband and constant group delay across the passband (i.e., square response). In practice however, the slope of the passband edge is finite and in most cases the group delay varies across the passband leading to phase distortion (i.e., dispersion).
Many optical filters used in WDM systems are inherently dispersive, which implies a nonlinear phase response. Furthermore, the amplitude and phase response are a Hilbert transform pair and therefore one uniquely determines the other. Such filters are known as minimum phase filters. It is important for the configuration of WDM systems to be able to characterize the phase and amplitude of such filters. The phase response of a filter is responsible for total time delay through the filter (first derivative of the spectral phase) and dispersion-induced pulse distortion (second and higher derivatives of the spectral phase response). However, while the amplitude response of filters is well understood and has received much attention, the phase response has only recently been investigated in the context of communication systems. While dispersion effects and dispersion compensation in optical fiber systems has been an active area of research, the dispersion associated with the filtering elements in the system have not been studied in detail.